Electrical apparatus



July 25, 1950 A. ALFOQD 2,516,500

' ELECTRICAL APPARATUS Filed March 26'. 1945 INVENTOR ANDREW ALFORDVyMWQMV ATTORNEY Patented July 25, 1950 UNITED: STATES. PATENT OFFICE;

ELECTRICAL APPARATUS.

Andrew Alford, Cambridge Mass assignor to the United States 'of Americaas represented:

by thesecretary of War Application March 26, 1946, Serial No. 657,143

apparatus, and particularly it-relates toan R. F.

antenna for aircraft installation which is. horizontally' polarized andhas radiation patterns:

in horizontal planes which are free of deep nulls.

For numerous aircraft installations it is desirable to employ antennashaving horizontal polarization and azimuth coverage in: all directions.A type of military operation in which an antenna. having thesecharacteristics is needed is airborne jamming Though sometimesthere is apreference for good forward coverage, the J'ammer usually shouldtransmit its signals in. all. directions. Moreover civilian. aviationequip?- ment such as communication apparatus and navigation aids mightalso require-such an antenna since blind spots in their coverage wouldconstitute a flying hazard.

In aircraft antennas installations a problem arises becauseradiatingelementsmust of necessity be quite close to reflectivesurfacesof the plane andthis can produce marked asymmetry in radiationpatterns; This asymmetry frequently unpredictableandmay causeundesirable directivity or blind spots. Besides this problem, theantennas themselves have shortcomings. Many antennas having desirablecharacteristics such as small size, low input impedance, good structuralstrength, and simple'construction, have horizontal. radiation patternswith deep nulls. A dipole, as a simple example of an otherwise desirablehorizontal radiator, has pronounced nulls in its horizontalpattern.

Prior art has already used: amodified simple dipole, the two'radiatinghalves. of which are made to meet as same angle other than=180. Such anantenna. is sometimes called. a nl" antenna, since its two radiatingelements meet-so as to form a letter V. V antennas have been installedin aircraft as singlecomplete units. They have been installed. either-infrontabove, below, or. to one of the two sides of the fuselage.Installation on either'side usually resulted in unpredictable asymmetry;installation above caused R. F. energy tomove predominantly away fromthe ground; installation 'belowinterfered with landing; and installationin front favored forward radiation.

It is'an object'of this inventionto provide a modified V dipole aircraftantenna installation 5 Claims. ('01. 25033.65)

Generally this invention comprises a V typeof dipole with its tworadiating elementsphysically' set so far apart that the fuselage of theplane fits between them. In this installation" these two radiatingelements protrude from said fuselage and are supported withrespectto'said' fuselage by supporting means physically con-- nected tothe fuselage.

Other objects, features and advantages of this invention will suggestthemselves to those skilled in'the art and will become apparent from thefollowing description of the invention taken in connection with thesingle figure in theaccom-- panying drawingin which the figure is aniso-- metric view of a fragment of an aircraft fuselage showing'theposition of thevarious elements of the present invention with respect tothe fuselage.

Referring now more particularly to the'figure','-the.antenna itself maybe considered as consisting of two radiating elements I and 2, eachofwhich should. have an eifective electricallength .near' to one. quarterwavelength corre-'- sponding to'the'm'idpoi'nt of the operating fre--quency band; Because of the broad bandafe'atures including in thisembodiment, it' is not al-" waysnecessary'that'elements l and 2 beexactly a quarterwavelength and, therefore, they will: besuitable for'use over relatively large frequency' bands. Fuselage fragment .3 hastwo openings 4 and 5 respectively which are largeenough' to permitelements I and 2 to protrude through the fuselage without beingelectrically grounded against the metallic skin .of the aircraft. An:in-

. sulatingbushing may be inserted to fill the space between elements 1and 2 and the perimeters-of' the respective'openings 3 and 5 inthe'aircraft' fuselage.

The elements I and 2 may be supportedby brackets inside the fuselage andthese brackets may be of any conventional type made in accordance with:ordinary design principles. It is merely desirable that they haveadequate strength to withstand air resistance-during flight and thatthey should be adapted to electrically insulate elements l and 2 fromthe structure ofthe aircraft. Balun 6 is a conventional unbalanced tobalanceR; F. transformer and may be designed to be broad handed inaccordance with conventional principles. Feed line I comes from atransmitter which is not shown and may be a low impedance cable of aconventional type: Lead-in lines 8 and 9 are similar cables whichconnect the output terminals-of Balun .5." to outer :conductive layersrespectively forming com.

. ductive skin .portions .of elements i and 2... The:

2,51e,soo

connection of lead-in line 8 to element l is made between innerconductor N! of line 8 and a conventional dog-ear tab H which is joinedto the conductive skin of element I. Lines 8 and 9 have the ends oftheir outer conductors electrically connected to the conductive exteriorskin of the aircraft ator near the point where elements I and 2respectively emerge from openings 4 and 5 respectively.

Transformer tab ll serves simply to avoid a marked discontinuity at thispoint where inner conductor H] which is physically small joins said skinwhich is physically large. The corresponding connection between lead-inline 9 and element 2 is not shown in the drawing. Lead-in lines 8 and 9should be kept as short as possible for the purpose of avoidingexcessive attenuation losses. However, except for this considerationthey are not of any critical length, but should be of equal length.

In operation, Balun 6 provides balanced R. F. energy for this antennaeven though an unbalanced transmission line is used to feed power fromthe transmitter. If elements I and 2 were connected with their endsclose together, and with each end connected to one of the outputreceptacles of Balun 5, they would comprise a dipole. Furthermore, ifthey were tilted back so as to form a V between them, they would thenform a V type of dipole and this dipole would I have the desiredradiation characteristics. The use of lead-in lines 8 and 9 permitselements I and 2 to act in substantially the same manner while at thesame time their ends are not close together but are, instead, separatedby the fuselage of the aircraft. Inasmuch as lines 8 and 9 are of equallength and are fed with 180 out of phase through the action of Balun 6,elements I and 2 are fed 180 out of phase despite the distance betweenthem. In general practice the two radiating elements forming V dipolesare made to meet each other at an angle of about 90. It has been foundthat embodiments of this invention installed on Army B24D type aircraftworked well where the radiating elements were tilted back to form 50angles with the axis of the fuselage. In these 324D installations theseelements were also tilted downward, somewhat, so that they did notexactly lie in the same horizontal plane.

If elements I and 2 are made to be fairly thick with respect to theirlengths, there are several desirable results: a good impedance match ofeach with its feed line; increased operating band width; and closercoupling to space. However, if the ratio of thickness to length exceeds1 to 10, another problem arises. The incident traveling wave moving onthe thick element is so attenuated in its travel along the length of theelement. due to very close coupling to space, that little energy is leftfor the reflected wave and thus the levels of the two are markedlydifferent. Therefore a new form of asymmetry is introduced and the mainlobes of each element point off at angles other than 90 to the axes ofthese main elements. To overcome this it becomes necessary to modify theangle of tilt, of the element itself. Generally speaking, for aninstallation of the kind shown herein, the angle at which each elementintersects lines parallel to the axis of the fuselage will be increasedabove said 50", if elements I and 2 were to be made very thick.

Due to the complexity of the reflective areas offered by the surfaces ofdifferent aircraft trial and error methods are needed when thisinvention is first applied to a given aircraft. However these antennashave been installed in many types of military aircraft and have usuallyacted in a predictable and satisfactory manner. The antenna herein setforth will radiate energy with horizontal polarization and with suchenergy distribution that the radiation patterns in horizontal planes arefree of deep nulls.

This antenna system can be looked upon as an array consisting of twomodified antenna stubs, modified in that they are oblique, rather thanperpendicular, to their respective reflecting planes. The axes of thesemodified stubs are 00- planar and, if extended, meet at an angle ofnearly The reflecting planes consist of parts of the fuselage; aresubstantially parallel to each other; and are separated from each otherby the width of the aircraft. The two modified stubs extend from sidesof said parallel reflecting planes which face away from each other andare fed R. F. energy out of phase. The expression, antenna stub, as usedherein, is intended to refer to an R. F. radiating element which is nearto one quarter wavelength long and extends substantially perpendicularlyfrom an associated reflecting plane.

It is also obvious that the installation is simple and lends itself touse in almost any conceivable type of aircraft.

It is obvious that elements I and 2 can be formed in any number of wayswithout departing from the spirit of the invention. They may bestream-lined but do not have to be; they may be either thick or thin;they may be composed of sheet metal, or solid metal, or ofnon-conductive core material with a metallic, or other conductive skin.It is also obvious that a Balun need not be employed at all, and thatthe two radiating elements can be fed directly from the conductors of abalanced pair.

As has been described above many departures from the embodimentprecisely set forth herein can exist without departing from the spiritand scope of this invention.

Accordingly I claim all such departures as fall fairly within the spiritand scope of this invention as herein disclosed and as claimed in theappended claims.

What is claimed is:

1. An antenna array including two radiating elements of a dipole antennaand reflecting surfaces therefor, said elements being obliquely disposedwith respect to said reflecting surfaces and mounted thereon, saidreflecting surfaces being portions of opposite sides of a structuresupporting said array, said reflecting surfaces being separated by anappreciable distance as measured in terms of the length of saidelements, said elements extending from said sides of said supportingstructure so as to point generally away from each other, said elementshaving axes which are substantially coplanar and which intersect saidreflecting surfaces at angles at least as great as 45, and feed meansfor exciting said elements with radio frequency energy 180 out of phase.

2. An antenna array as in claim 1 in which the points of intersection ofsaid axes with said refleeting surfaces are so located on the oppositesides of said supporting structure that a line joining said points wouldform substantially equal angles with both of said surfaces.

3. An antenna array including two radiatingelements of a dipole andreflecting surfaces therefore, said elements being disposed obliquelywith respect to said surfaces rather than .perpendicularly, saidsurfaces comprising portions of opposite exterior surfaces of anairplane fuselage on which said array is mounted, said elementsextending outward from said opposite sides of said fuselage so that theytilt backward toward the rear portion of said fuselage, said elementshaving axes which are substantially coplanar and which intersect saidsurfaces so that the angles measurable between said axes and saidsurfaces are at least as great as 45, said elements being so located onthe opposite sides of said fuselage that a line joining the points wheretheir axes intersect said surfaces would form substantially equal angleswith said surfaces, and feed means for exciting said elements with radiofrequency energy 180 out of phase.

4. An antenna array including two longitudinal radiating elements ofsubstantially equal size and reflecting surfaces therefcr, said elementsbeing disposed obliquely with respect to said reflecting surfaces ratherthan perpendicularly and mounted thereon, said reflecting surfaces beingseparated by an appreciable distance as measured in terms of the lengthof said elements, said elements extending from said reflecting surfacesso as to point away from each other, said elements having axes which aresubstantially coplanar and which meet their respective reflectingsurfaces so obliquely that the angles measured between said axes andsaid respective reflecting surfaces are at least as great as 45, andfeed means for exciting said elements with radio frequency energy 180out of phase.

5. A broad band antenna array having two reflecting surfaces, saidreflecting surfaces comprising opposite sides of an airplane fuselage, aradiating element mounted obliquely on each of said reflecting surfaces,said radiating elements being thick in cross section relative to thelengths thereof, said radiating elements being separated from each otheran appreciable distance as measured in terms of the wavelength of thedesired operating frequency the axes of said radiating elements beingcoplanar and forming angles at least as great as with said reflectingsurfaces, and means to feed said radiating elements out of phase withrespect to each other.

ANDREW ALFORD.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,942,594 Edwards et al. Jan. 9,1934 2,155,652 Gothe et al Apr. 25, 1939 2,216,708 Kolster Oct. 1, 19402,224,898 Carter Dec. 17, 1940 2,235,015 Eggers May 18, 1941 2,352,977Scheldorf July 4, 1944 2,368,663 Kandoian Feb. 6, 1945 2,370,628 AlfordMar. 6, 1945 2,425,303 Carter Aug. 12, 1947 2,452,073 Schivley et alOct. 26, 1948

